Due to actualization of environmental problems in recent years, hybrid cars and electric cars have spread. Further, as an air conditioner which is applicable to such a vehicle, there has been developed an air conditioner which includes a refrigerant circuit constituted of a compressor to compress and discharge a refrigerant, a radiator disposed in an air flow passage of an HVAC unit to let the refrigerant radiate heat, a heat absorber disposed in the air flow passage to let the refrigerant absorb heat, an outdoor heat exchanger disposed outside a vehicle interior to let the refrigerant radiate heat, and others, and which lets the refrigerant discharged from the compressor radiate heat in the radiator to heat the air to be supplied from an indoor blower, thereby heating the vehicle interior (e.g., see Patent Document 1).
Furthermore, in Patent Document 1, the air conditioner has a constitution where an electric heater is provided in the air flow passage to exert a heating capability of the electric heater in addition to that of the radiator. In particular, for the electric car, there is also a constitution where the radiator does not perform such heating (the refrigerant circuit is constituted only for cooling) and the heating is performed only by the electric heater to which power is supplied from a battery.
Here, the HVAC unit including the air flow passage therein is usually formed of a hard resin, and when a temperature of the electric heater abnormally heightens from any cause, there is the risk that the HVAC unit (the air flow passage) is deformed or molten by the heat of the electric heater. To avoid the risk, heretofore, a plurality of temperature sensors have been attached to the electric heater, and there has been provided a protective function of limiting energization of the electric heater and finally stopping the energization, when the temperature (a detected value) of the electric heater which is detected by each temperature sensor abnormally rises.